JavaScript Syntax

JavaScript syntax is the set of rules that determine how a valid JavaScript program is written and structured. Unlike languages such as Java or C#, JavaScript does not require a surrounding class or explicit entry point — a script is simply a sequence of statements that the JavaScript engine reads and executes from top to bottom.

Because JavaScript is loosely structured and forgiving in places (like optional semicolons), it is especially important to understand exactly what the engine does with your code, rather than relying on "it seems to work." Small syntax habits formed early — statement structure, semicolon usage, and commenting — carry through every other topic in the language.

Why it matters

JavaScript inserts missing semicolons for you, and in one situation it inserts one where you did not want it: directly after return. The code is valid, runs without a syntax error and silently returns undefined. Knowing the basic rules of statements and semicolons saves you from this class of quiet bug.

Statements and Expressions

A JavaScript program is made of statements — instructions that the engine executes, such as declaring a variable, running a loop, or calling a function. An expression is anything that produces a value, such as 2 + 2 or "hello".toUpperCase(). Statements are often built from expressions: let total = price * quantity; is a statement containing the expression price * quantity.

• Statements end a "step" of execution: declarations, assignments, conditionals, loops, function calls.
• Expressions always evaluate to a value and can be nested inside other expressions or statements.
• Blocks of statements are grouped with curly braces { }, used by functions, loops, and conditionals.

Semicolons and Automatic Semicolon Insertion (ASI)

JavaScript statements are conventionally ended with a semicolon, but the language includes a feature called Automatic Semicolon Insertion (ASI), which inserts semicolons for you in certain cases when you omit them. ASI is convenient but has sharp edges: it does not always insert a semicolon where you expect, particularly with return statements followed by a newline, or lines starting with ( or [ that can be misread as a continuation of the previous line. The safest professional practice is to write semicolons explicitly rather than rely on ASI.

Comments

JavaScript supports single-line comments starting with //, which run to the end of the line, and multi-line comments wrapped in /* ... */, which can span several lines. Comments are ignored entirely by the engine and exist only to communicate intent to human readers.

Case Sensitivity and Whitespace

JavaScript is case-sensitive, so myValue and myvalue are different identifiers. Whitespace (spaces, tabs, newlines) is generally insignificant between tokens and is used only for readability, except inside string literals where it is preserved exactly as written.

Examples

function getValue() {
  return
  {
    value: 42
  };
}

console.log(getValue()); // ASI inserts a semicolon right after "return"

function getValueFixed() {
  return {
    value: 42
  };
}

console.log(getValueFixed());

undefined
{ value: 42 }

// Calculate the total price for an order
const price = 25;
const quantity = 3;

/*
  Multiplying price by quantity gives the subtotal.
  Tax is added separately in a later step.
*/
const subtotal = price * quantity;
console.log(subtotal);

75

Exercise

The function is meant to return an object, but because the object starts on the line after return, JavaScript ends the statement there and the function returns undefined. Fix it.

Expected output: Asha

function getUser() {
  return
  {
    name: "Asha"
  };
}

console.log(getUser().name);

function getUser() {
  return {
    name: "Asha"
  };
}

console.log(getUser().name);

Quick quiz

Interview questions